fix typo in python3_version
fix typo in python3_version

The function was using an undefined variable to show the version of
python3 being used.

Change-Id: Ibc956975d620ed5174de8823f9c202a680c56aaf
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

group-policy/devstack,openstack-dev/devstack,nawawi/openstack,noironetworks/devstack,dtroyer/devstack,noironetworks/devstack,openstack-dev/devstack,group-policy/devstack,dtroyer/devstack,nawawi/openstack